Theranostics 2019, Vol. 9, Issue 8 2129 Ivyspring International Publisher Theranostics 2019; 9(8): 2129-2142. doi: 10.7150/thno.31179 Research Paper Preclinical efficacy of hK2 targeted [177Lu]hu11B6 for prostate cancer theranostics Oskar Vilhelmsson Timmermand1, Jörgen Elgqvist2, Kai A. Beattie3, Anders Örbom1, Erik Larsson4, Sophie E. Eriksson1, Daniel L.J. Thorek5, Bradley J. Beattie6, Thuy A. Tran7,8, David Ulmert1,3,9*, Sven-Erik Strand1,4* 1. Division of Oncology and Pathology, Department of Clinical Sciences Lund, Lund University, Lund, Sweden 2. Department of Medical Physics and Biomedical Engineering, Sahlgrenska University Hospital, Gothenburg, Sweden 3. Molecular Pharmacology Program, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, NY 10065, USA 4. Division of Medical Radiation Physics, Department of Clinical Sciences Lund, Lund University, Lund, Sweden 5. Department of Radiology, Washington University School of Medicine, Saint Louis, MO, 63108, USA. 6. Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY 10065, USA. 7. Department of Radiopharmacy, Karolinska University Hospital, Stockholm, Sweden 8. Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden 9. Department of Molecular and Medical Pharmacology, David Geffen School of Medicine at University of California, Los Angeles (UCLA), CA, USA *Equal contribution Corresponding author: Division of Oncology, Department of Clinical Sciences Lund, Lund University, Barngatan 4, bottom floor, 221 85 Lund, Sweden. E-mail: [email protected] © Ivyspring International Publisher. This is an open access article distributed under the terms of the Creative Commons Attribution (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/). See http://ivyspring.com/terms for full terms and conditions. Received: 2018.11.02; Accepted: 2019.01.17; Published: 2019.04.06 Abstract Androgen ablating drugs increase life expectancy in men with metastatic prostate cancer, but resistance inevitably develops. In a majority of these recurrent tumors, the androgen axis is reactivated in the form of increased androgen receptor (AR) expression. Targeting proteins that are expressed as a down-stream effect of AR activity is a promising rationale for management of this disease. The humanized IgG1 antibody hu11B6 internalizes into prostate and prostate cancer (PCa) cells by binding to the catalytic cleft of human kallikrein 2 (hK2), a prostate specific enzyme governed by the AR-pathway. In a previous study, hu11B6 conjugated with Actinium-225 (225Ac), a high linear energy transfer (LET) radionuclide, was shown to generate an AR-upregulation driven feed-forward mechanism that is believed to enhance therapeutic efficacy. We assessed the efficacy of hu11B6 labeled with a low LET beta-emitter, Lutetium-177 (177Lu) and investigated whether similar tumor killing and AR-enhancement is produced. Moreover, single-photon emission computed tomography (SPECT) imaging of 177Lu is quantitatively accurate and can be used to perform treatment planning. [177Lu]hu11B6 therefore has significant potential as a theranostic agent. Materials and Methods: Subcutaneous PCa xenografts (LNCaP s.c.) were grown in male mice. Biokinetics at 4-336 h post injection and uptake as a function of the amount of hu11B6 injected at 72 h were studied. Over a 30 to 120-day treatment period the therapeutic efficacy of different activities of [177Lu]hu11B6 were assessed by volumetric tumor measurements, blood cell counts, molecular analysis of the tumor as well as SPECT/CT imaging. Organ specific mean absorbed doses were calculated, using a MIRD-scheme, based on biokinetic data and rodent specific S-factors from a modified MOBY phantom. Tumor tissues of treated xenografts were immunohistochemically (IHC) stained for Ki-67 (proliferation) and AR, SA-β-gal activity (senescence) and analyzed by digital autoradiography (DAR). Results: Organ-to-blood and tumor-to-blood ratios were independent of hu11B6 specific activity except for the highest amount of antibody (150 µg). Tumor accumulation of [177Lu]hu11B6 peaked at 168 h with a specific uptake of 29 ± 9.1 percent injected activity per gram (%IA/g) and low accumulation in normal organs except in the submandibular gland (15 ± 4.5 %IA/g), attributed to a cross-reaction with mice kallikreins in this organ, was seen. However, SPECT imaging with therapeutic amounts of [177Lu]hu11B6 revealed no peak in tumor accumulation at 7 d, probably due to cellular retention of 177Lu and decreasing tumor volumes. For [177Lu]hu11B6 treated mice, tumor decrements of up to 4/5 of the initial tumor http://www.thno.org Theranostics 2019, Vol. 9, Issue 8 2130 volume and reversible myelotoxicity with a nadir at 12 d were observed after a single injection. Tumor volume reduction correlated with injected activity and the absorbed dose. IHC revealed retained expression of AR throughout treatment and that Ki-67 staining reached a nadir at 9-14 d which coincided with high SA- β-gal activity (14 d). Quantification of nuclei staining showed that Ki-67 expression correlated negatively with activity uptake. AR expression levels in cells surviving therapy compared to previous timepoints and to controls at 30 d were significantly increased (p = 0.017). Conclusions: This study shows that hu11B6 labeled with the low LET beta-emitting radionuclide 177Lu can deliver therapeutic absorbed doses to prostate cancer xenografts with transient hematological side-effects. The tumor response correlated with the absorbed dose both on a macro and a small scale dosimetric level. Analysis of AR staining showed that AR protein levels increased late in the study suggesting a therapeutic mechanism, a feed forward mechanism coupled to AR driven response to DNA damage or clonal lineage selection, similar to that reported in high LET alpha-particle therapy using 225Ac labeled hu11B6, however emerging at a later timepoint. Introduction Castration resistant prostate cancer (CRPC) is for the catalytic cleft of active hK2. To date, we have associated with poor prognosis and is often shown that hu11B6 specifically internalizes into hK2 characterized by sustained AR-signaling over the expressing cells in vivo through a mechanism that entire course of disease progression. In fact, relies on the intact variable region, and on the ‘castrate-resistant’ reflects continued androgen fragment crystallizable (Fc) region’s ability to bind to dependence, rather than independence [1]. Thus, the neonatal Fc receptor (FcRn) [13]. much of the effort spent developing new In a previous study, following therapy with pharmacological compounds has been focused on hu11B6 labeled with the high LET alpha-particle either AR blockade or androgen synthesis deprivation emitting radionuclide Actinium-225 (225Ac), we with drugs such as apalutamide and abiraterone. reported an increase in AR-activity and hK2 However, despite interventions with these drugs (and production in cells as an effect of DNA repair [14]. often following an initial response), some form of The AR flair phenomenon has initially been reported reactivation of the AR pathway inevitably occurs. The based on Caesium-137 (137Cs) external beam radiation radiotherapeutic we consider here follows an [15, 16], and to our knowledge it has not been alternate line of reasoning. Rather than attempting to previously demonstrated that low LET beta-emissions disrupt the AR-signaling, we take advantage of its generate the same effect. High LET alpha-particles presence by targeting potent cytotoxic agents to produce irreparable damage, i.e. DNA double strand proteins resultant from AR pathway activation [2]. breaks (DSB), when traversing the nucleus compared The use of radioimmunotherapy (RIT) has been to low LET beta-particles which to a higher degree shown to be a successful treatment of liquid tumors indirectly damage the cell via the creation of reactive using anti-CD20 and CD33 antibodies [2,3]. oxygen species [17,18]. This oxidative stress results in Radionuclide therapy of CRPC patients using prostate DNA single strand breaks (SSB) and a lower specific membrane antigen (PSMA) targeting frequency of DSB. However, the occurrence of an AR modalities are promising but might have severe flair phenomenon caused by low LET beta-particles dose-limitations due to high expression and uptake in could potentially result in enhanced treatment off-target organs such as salivary glands and kidneys efficacies relating to a feed-forward mechanism. This [4-7]. is of particular interest since various low LET Human kallikrein related peptidase 2 (hK2) is a β-emitters are frequently used in radiotherapeutic serine protease that is exclusively found in prostate agents. tissues (both healthy and malignantly derived) and is The overarching aim of the current study was to known to have increased expression with the severity gather dosimetric, therapeutic and radiobiological of the disease [8-11]. KLK2, the gene encoding hK2, is data that will guide our upcoming clinical translation regulated by the AR pathway. In a manner similar to of radiolabeled hu11B6. In this study we investigated that of the closely related kallikrein, Prostate specific whole body and small-scale tumor dosimetry in an antigen (PSA), active hK2 is retrograde released into animal model, as well as therapeutic and biological the blood where it is immediately deactivated by effects of hu11B6 labeled with Lutetium-177 (177Lu), a abundant protease inhibitors [12]. We have